1. Field of the Invention
The present invention relates to a thin transmission line configured to transmit a high-frequency signal and an electronic device including the transmission line.
2. Description of the Related Art
At present it attracts attention to use such a flat cable as illustrated in Japanese Patent No. 4962660 as a high-frequency transmission line for transmitting a high-frequency signal. While the flat cable is larger in width than a coaxial cable, it is possible to make the flat cable thin. Therefore, in particular the flat cable is useful in a case where there is only a thin clearance gap within a terminal housing. In addition, since a dielectric body having flexibility is used as a base material, the flat cable has flexibility and can easily adopt curved or flexed routing.
The flat cable described in Japanese Patent No. 4962660 has a triplate-type strip line structure, as a basic structure.
Such a flat cable as illustrated in Japanese Patent No. 4962660 includes a substantially plate-shaped dielectric body having flexibility and an insulation property. The dielectric body has a substantially elongated shape that extends in a substantially straight line. In one of two surfaces of the dielectric body perpendicular or substantially perpendicular to the thickness direction thereof, a reference ground conductor is arranged. The reference ground conductor is a so-called substantially flat plate-shaped conductor pattern covering nearly the entire surface of the second surface of a base material sheet.
In the other surface facing one of two surfaces of the base material sheet, an auxiliary ground conductor is arranged. The auxiliary ground conductor includes two substantially elongated conductors whose shapes each extend in a longitudinal direction and which are located in two respective end portions thereof in a width direction perpendicular or substantially perpendicular to the longitudinal direction and a thickness direction. The two substantially elongated conductors are connected to each other by bridge conductors each forming a shape that extends in the width direction. The bridge conductors are disposed in the longitudinal direction with predetermined distances therebetween. As a result, the auxiliary ground conductor forms a shape in which an array of opening portions of predetermined opening lengths is formed in the longitudinal direction.
Roughly in the middle portion of the dielectric body in the thickness direction thereof, a signal conductor is formed with a predetermined width and a predetermined thickness. The signal conductor forms a substantially elongated shape that extends in a direction parallel to the substantially elongated conductor portion of the auxiliary ground conductor and the reference ground conductor. The signal conductor is formed roughly in the middle portion of the dielectric body in the width direction thereof.
According to such a configuration, when the flat cable is viewed in plan (viewed from a direction perpendicular or substantially perpendicular to one of two surfaces and the other surface), the signal conductor only overlaps with the bridge conductors of the auxiliary ground conductor and is disposed so that the other regions thereof are located within the opening portions.
However, at present there is required a transmission line capable of transmitting two kinds of high-frequency signals in parallel. In a case where the flat cable described in Japanese Patent No. 4962660, flat cables whose number corresponds to the number of high-frequency signals to be transmitted in parallel are required. Accordingly, it is necessary to arrange and dispose these elements in parallel, and an amount of space is needed.
In addition, in a case where the structures of the transmission lines described in Japanese Patent No. 4962660 are arranged and disposed in parallel in one dielectric body, the positions of the opening portions in a transmission direction coincide with each other between transmission lines adjacent to each other. In this case, positions at which emissions of electric fields or magnetic fields generated by high-frequency signals transmitted through the respective transmission lines are large coincide with each other between the transmission lines adjacent to each other. Accordingly, isolation between the transmission lines adjacent to each other is not secured and crosstalk between the high-frequency signals transmitted through the respective transmission lines turns out to be generated.
If, in particular, transmission lines including flat cables are intended to be downsized, a distance between transmission lines adjacent to each other is decreased, isolation is further reduced, and crosstalk is easily generated.